Fatigue behavior of metals is reviewed with particular emphasis on those properties and parameters which relate to cyclic deformation resistance. Representative data for aluminum-, titanium-, and nickel-base alloys and steels strengthened by various processes are presented to illustrate procedures for characterizing cycle-dependent deformation and fracture behavior.
The nature and extent of cyclically induced changes in deformation resistance are conveniently described in terms of a cyclic stress-strain curve. A metal's monotonic strain hardening behavior provides an indication of cyclic stability. Fracture behavior is characterized by simple relations in terms of stress resistance, plastic strain resistance, and total strain resistance. True monotonic fracture strength and ductility can be related to fatigue strength and ductility, thus providing useful approximations of life behavior. Indications of notched fatigue resistance can be gained from smooth specimen data through consideration of local stress-strain response. Finally, the utility of such material behavior considerations in arriving at the proper combination of properties to maximize the fatigue resistance of a metal under specified conditions is discussed.
Procedures are presented for analyzing the cyclic stability and influence on fatigue resistance of residual stress patterns arising from mechanical and thermal surface processing treatments such as shot peening and induction hardening. Cyclic properties and behavioral trends developed using smooth, axial specimens of steels simulating the various microstructures found in surface processed componentry are used to develop criteria to predict cyclic stability. the rate of relaxation for prescribed straining levels, the failure initiation point (surface or subsurface), and expected fatigue lifetime. The validity of the approach is verified using experimental data from the literature. Finally, the incorporation of these procedures in modern computer-based fatigue analysis routines, and opportunities for further enhancements, are discussed.
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